What is the number in corner of the 3D view?
It shows what frame on a timeline you already are. It is very useful if you e.g. maximized the window with View -> Toggle Maximize Area (Shift+Space) and don't see your time cursor (the green vertical line) on the Timeline. Please see how to navigate it btw (What are the hotkeys to navigate the timeline?).



It's also very helpful as it shows what frame on a timeline the animation keyframe is inserted (the number changes the color from white to orange then).